The present invention relates to electroluminescent (EL) lamps and more particularly to an improved EL lamp structure having fewer required printed component layers thereby improving manufacturing cycle time and product quality.
EL lamps are basically devices that convert electrical energy into light. AC current is passed between two electrodes insulated from each other and having a phosphorous material placed therebetween. Electrons in the phosphorous material are excited to a higher energy level by an electric field created between the two electrodes during the first quarter cycle of the AC voltage. During the second quarter cycle of the AC voltage, the applied field again approaches zero. This causes the electrons to return to their normal unexcited state. Excess energy is released in the form of a photon of light when these electrons return to their normal unexcited state. This process is repeated for the negative half of the AC cycle. Thus, light is emitted twice for each full cycle (Hz). Various properties of the emitted light can be controlled by varying this frequency, as well as the applied AC voltage. In general, the brightness of the EL lamp increases with increased voltage and frequency.
Prior art EL lamps typically comprise numerous component layers. At the light-emitting side of an EL lamp (typically the top) is a front electrode, which is typically made of a transparent, conductive indium tin oxide (ITO) layer and a silver bus bar to deliver maximum and uniform power to the ITO. Below the ITO/bus bar layers is a layer of phosphor, followed by a dielectric insulating layer and a rear electrode layer. All of these layers are typically disposed on a flexible or rigid substrate, which is typically polyester. In some prior art EL lamps, the ITO layer is sputtered on a polyester film, which acts as a flexible substrate. A relatively thick polyester film, typically four or more mils thick, is necessary because the rigidity is required for the screen printing of the layers. The EL lamp construction may also include a top film laminate or coating to protect the component layers of the EL lamp construction.
The component layers of an EL lamp are typically constructed from a variety of materials, including films and electrodes, polymeric films, printed layers, encapsulants, epoxies, coatings or combinations thereof if these layers are printed, they.are normally printed by means of a flat bed screen method and are then batch dried, except for the base substrate and top film laminate. Some of the required layers must be printed more than once in order to assure proper thickness. For example, the dielectric material needs sufficient thickness to prevent pinholes or voids, which may cause shorting between the electrodes. On the other hand, the dielectric layer is prone to cracking when multiple layers are printed one over the other. Thus, control over the printing process for the dielectric layer is extremely important. If the dielectric is too thick, the required operating power and frequency to achieve a given brightness must be increased. Also the chances of cracking will be increased; thus, consistent dielectric thickness in production of EL lamps is important to ensure consistent lamp brightness across a given production run of lamps.
Another limitation of a multilayer printed dielectric is the effect it has on the quality of the other component layers that are printed thereon. For example, the printed phosphor layer must be smooth and consistent to ensure uniform lighting from the excited phosphor. If the multilayer printed dielectric layer is inconsistent, then the phosphor layer printed on the dielectric layer will also be inconsistent. An inconsistent printed dielectric layer will also affect other subsequently printed layers, including the transparent electrode layer. Thus, a smooth dielectric layer is important to ensure the quality of all the subsequent printed layers and ultimately the quality of the EL lamp.
Another drawback of utilizing multi-printed layers is the effect on production cycle time. Each of the printed layers of the EL lamp structure, with the exception of the base substrate and top film laminate, has to be printed and then dried before another printed layer is applied. This is a very time-consuming and expensive process, especially for printing the multilayered dielectric.
It is therefore an object of the present invention to provide an EL lamp structure that reduces the number of printed layers by utilizing a dielectric film in lieu of a printed dielectric layer, thus reducing the printing and drying time in the production process and increasing the reliability and quality of the EL lamp.
It is also an object of the present invention to provide an EL lamp structure that utilizes a dielectric film in lieu of a printed dielectric layer, thus eliminating the need to print on top of the thick printed dielectric layer and thereby improving the print quality of the phosphor and transparent electrode layers.
It is another object of the present invention to provide an EL lamp structure that utilizes a dielectric film in lieu of a printed dielectric layer, thus greatly reducing the possibility of shorting between the electrodes of the EL lamp.
It is a further object of the present invention to provide an alternate EL lamp structure that further reduces the required number of component layers by utilizing a metalized film that acts as both a rear electrode and a dielectric layer, thus even further reducing the printing and drying time in the production process and increasing the reliability and quality of the EL lamp.
These and other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.
The present invention is an improved EL lamp structure having a reduced number of structural or component layers and therefore fewer printed component layers. The EL lamp utilizes a flexible dielectric film, such as polypropylene, polyethylene or polyethylene terephthalate (PET), that acts as a combination dielectric layer and structural substrate for the remaining layers of the EL lamp structure. The flexible dielectric film reduces the need for a separate dielectric layer and substrate layer. Furthermore, the flexible dielectric film eliminates the need for several printed dielectric layers, thus reducing production time and the occurrence of manufacturing defects during the printing process.
The remaining structure of the EL lamp is applied to the flexible dielectric film substrate. A phosphor layer is printed on the top side of the dielectric film. Since a dielectric film is being used, the print quality of the phosphor printed upon the smooth film surface will be more consistent than if the phosphor was printed on several layers of a printed dielectric ink. A transparent electrode layer, such as printable indium tin oxide (ITO), is printed on the phosphor layer. A front bus bar is then printed on the transparent electrode layer. The front bus bar is typically printed with silver or carbon ink. A rear electrode can be printed on the bottom surface of the dielectric film. The application of a top and/or bottom laminate, lacquer, or the like is optional and helps protect the EL lamp structure from adverse environmental conditions as well as protecting users from electrical hazards. A laminate or similar coating will particularly protect the phosphor layer from moisture damage.
In an alternate embodiment, a low cost commercially available flexible metalized film is used as a combination rear electrode, dielectric layer and substrate. This embodiment further reduces the number of printed component layers required in the EL lamp structure. Typical metalized film has aluminum, copper, or other metallic conductive material deposited on one side of the film by vapor deposition, sputtering, plating, printing or other metallic deposit techniques known in the art. The deposited metallic layer acts as the rear electrode and the film material, such as a polyester resin, acts as the dielectric layer. The film also acts as a substrate for application of the remaining printed component layers.